1. Field of the invention
This invention relates generally to the field of high frequency electronic ballasts for gas discharge lamps, and more particularly to those ballasts which permit lamp dimming through the use of a phase controlled dimmer (triac dimmer) and which allow wide-range dimming (100% - 1% or less) for fluorescent lamps.
2. Description of the prior art
The basic operating principle of a gas discharge lamp was described by William E. Forsythe and Elliot Q. Adams in 1948 in a book entitled "Fluorescent and Other Gaseous Discharge Lamps," and later by John F. Waymouth in 1971 in a book entitled "Electric Discharge Lamps."
The basic principles for dimming gas discharge lamps are well known. The prior art dimmer circuits all suffer from certain disadvantages as described below. For example, a circuit is disclosed in U.S. Pat. No. 4,392,087 to Zansky which is capable of operating lamps at a dimming level about 10% of full brightness. The circuit has an inverter, operates with an AC voltage feedback base driver, and has an output transformer with a fixed leakage inductor. The tuning capacitor is connected to a secondary winding and resonated with a leakage inductor. This circuit has a number of inefficiencies. The filament voltage is not increased at low brightness levels which significantly reduces lamp life. The inverter does not switch under zero voltage or zero current which results in a relatively high switching loss. Also the base drive AC voltage is reduced when the lamp brightness is reduced. Consequently, the transistors are not well saturated and the inverter's switching on loss is relatively high. In other words, the circuit is very inefficient. Because this circuit is not operating in the constant current mode, this ballast will not be able dim down to 1% of full brightness.
Another prior dimmer circuit is disclosed in U.S. Pat. No. 4,001,637 to Gray. This circuit is comprised of a phase-controlled dimmer with a non-dissipative current limiting capacitor and a DC current smoothing filter. This dimmer circuit suffers from certain disadvantages in that it is only capable of operating ballasts with series inductors or auto-transformers.
Yet another prior circuit is disclosed in U.S. Pat. No. 4,704,561 to Dietl. This circuit is configured as a half-bridge inverter with a series resonate tank load. The output transistor's base drive current is actively controlled by another transistor. This is a costly feature. The inverter does not operate under zero current or zero voltage. Therefore it is not very efficient. The inverter needs an additional pulse generator to control the light output which makes this circuit impractical to use.
Another prior circuit is disclosed in U.S. Pat. No. 5,004,959 to Nilssen. This ballast comprises a self-oscillating, half-bridge inverter with a saturable drive transformer. The lamp is in parallel with a series tank circuit. By changing the position of a permanent magnet within the saturation drive transformer, the operating frequency of the inverter is changed. By adjusting the operating frequency, the lamp current and, consequently, the brightness level is adjusted. This circuit is disadvantageous because it requires a mechanical link to dim the light and it is incapable of being remotely controlled.
Another prior circuit is disclosed in U.S. Pat. No. 4,859,914 to Summa. This circuit has a single transistor self-oscillator and a TRIAC open circuit protector. This circuit dims by a photo cell (automatically) or by an external potentiometer with additional control wires. The input choke has a single-winding inductor in parallel with a capacitor. This circuit is disadvantageous because it requires a special fixture for the photocell to operate properly and it requires an extra control line which makes the installation of the ballast complicated and expensive.
A last prior circuit is disclosed in U.S. Pat. No. 5,001,386 to Sullivan et al. This circuit has a pulse duration modulation section to drive a half-bridge inverter with a series tank circuit. By adjusting the duration of the pulse, the brightness level is controlled down to 1%. However, this circuit requires an external control line which makes the installation of the ballast complicated and expensive. This circuit also uses a DC current to bias the lamps for reducing the striations in the lamp at the low brightness levels.
As can be ascertained from the description of prior dimmable gas discharge lamps above, there is a need for a dimmable electronic ballast for gas discharge lamps which has a small number of components and is therefore inexpensive to produce and is highly efficient.
There is also a need for a dimmable electronic ballast for gas discharge lamps which exhibits superior performance characteristics over the prior circuits.
Additionally, there is a need for a dimmable electronic ballast which can be controlled by a traditional two wire, phase-controlled dimmer. There is an additional need for a dimmable electronic ballast with an improved power factor and with reduced radio frequency interference.
There is yet an additional need for a dimmable electronic ballast with a delayed starting feature and with overvoltage protection.